Flexible circuits are typically composed of at least one metal layer such as copper (Cu), nickel (Ni), tin (Sn), silver (Ag) or gold (Au) on a flexible polymeric film such as polyester (PET), polyimide (PI), or liquid crystal polymer (LCP). When the metal is attached to the polymer film without the use of an additional adhesive layer, the construction is often classified as a “two-layer” or “adhesiveless” flexible circuit. Generally, two-layer substrates consist of polymer films of about 12 to 75 microns thickness. A thin tie layer is usually employed to promote adhesion between the metal and the polymer film, to prevent or minimize corrosion at the metal/film interface, and to provide a diffusion barrier between the metal and the polymer film. For high performance applications, the primary metal layer is commonly copper while the film layer is polyimide.
Tie layers range from about 2 nm to about 500 nm thick and are formed by methods such as vacuum sputtering, vacuum evaporation, and chemical plating typically of chromium (Cr), nickel (Ni), cobalt (Co), molybdenum (Mo) or their related alloys. To be useful in high performance flexible circuit applications using copper as the metal, it should be possible to etch the tie layer in the presence of copper, so that the copper traces can be electrically isolated. Often this is a challenging requirement. Tie layers that are highly corrosion resistant are difficult to etch. Aggressive etchants that can attack the corrosion resistant tie layer can also significantly attack the copper. In addition, many non-corroding tie layers tend to passivate readily and are less active than copper. As a result, copper can inhibit the etching of the tie layer and make it difficult to remove, especially in fine pitch regions of a circuit.